Re: [PATCH v5 00/22] Readdir enhancements

[David Wysochanski <dwysocha@redhat.com>]

On Tue, Nov 10, 2020 at 4:48 PM <trondmy@kernel.org> wrote:
>
> From: Trond Myklebust <trond.myklebust@hammerspace.com>
>
> The following patch series performs a number of cleanups on the readdir
> code.
> It also adds support for 1MB readdir RPC calls on-the-wire, and modifies
> the caching code to ensure that we cache the entire contents of that
> 1MB call (instead of discarding the data that doesn't fit into a single
> page).
> For filesystems that use ordered readdir cookie schemes (e.g. XFS), it
> optimises searching for cookies in the client's page cache by skipping
> over pages that contain cookie values that are not in the range we are
> searching for.
> Finally, it improves scalability when dealing with very large
> directories by turning off caching when those directories are changing,
> so as to avoid the need for a linear search on the client of the entire
> directory when looking for the first entry pointed to by the current
> file descriptor offset.
>
> v2: Fix the handling of the NFSv3/v4 directory verifier.
> v3: Optimise searching when the readdir cookies are seen to be ordered.
> v4: Optimise performance for large directories that are changing.
>     Add in llseek dependency patches.
> v5: Integrate Olga's patch for the READDIR security label handling.
>     Record more entries in the uncached readdir case. Bump the max
>     number of pages to 512, but allocate them on demand in case the
>     readdir RPC call returns fewer entries.
>
> Olga Kornievskaia (1):
>   NFSv4.2: condition READDIR's mask for security label based on LSM
>     state
>
> Trond Myklebust (21):
>   NFS: Remove unnecessary inode locking in nfs_llseek_dir()
>   NFS: Remove unnecessary inode lock in nfs_fsync_dir()
>   NFS: Ensure contents of struct nfs_open_dir_context are consistent
>   NFS: Clean up readdir struct nfs_cache_array
>   NFS: Clean up nfs_readdir_page_filler()
>   NFS: Clean up directory array handling
>   NFS: Don't discard readdir results
>   NFS: Remove unnecessary kmap in nfs_readdir_xdr_to_array()
>   NFS: Replace kmap() with kmap_atomic() in nfs_readdir_search_array()
>   NFS: Simplify struct nfs_cache_array_entry
>   NFS: Support larger readdir buffers
>   NFS: More readdir cleanups
>   NFS: nfs_do_filldir() does not return a value
>   NFS: Reduce readdir stack usage
>   NFS: Cleanup to remove nfs_readdir_descriptor_t typedef
>   NFS: Allow the NFS generic code to pass in a verifier to readdir
>   NFS: Handle NFS4ERR_NOT_SAME and NFSERR_BADCOOKIE from readdir calls
>   NFS: Improve handling of directory verifiers
>   NFS: Optimisations for monotonically increasing readdir cookies
>   NFS: Reduce number of RPC calls when doing uncached readdir
>   NFS: Do uncached readdir when we're seeking a cookie in an empty page
>     cache
>
>  fs/nfs/client.c         |   4 +-
>  fs/nfs/dir.c            | 734 +++++++++++++++++++++++++---------------
>  fs/nfs/inode.c          |   7 -
>  fs/nfs/internal.h       |   6 -
>  fs/nfs/nfs3proc.c       |  35 +-
>  fs/nfs/nfs4proc.c       |  48 +--
>  fs/nfs/proc.c           |  18 +-
>  include/linux/nfs_fs.h  |   9 +-
>  include/linux/nfs_xdr.h |  17 +-
>  9 files changed, 541 insertions(+), 337 deletions(-)
>
> --
> 2.28.0
>

I wrote a test script and ran this patchset against 5.10-rc2 in a
variety of scenarios listing 1 million files and various directory
modification scenarios. The test checked ops, runtime and cookie
resets (via tcpdump).

All runs performed very well on all scenarios I threw at it on all NFS
versions (I tested 3, 4.0, 4.1, and 4.2) with fairly dramatic
improvements vs the same runs on 5.10-rc2.   One scenario I found
where a single 'ls -l' could take a long time (about 5 minutes, but
not unbounded time) was when the directory was being modified with
both file adds and removed, but this seemed due to other ops unrelated
to readdir performance.   A second scenario that this patchset does
not fix is the scenario where the directory is large enough to exceed
the acdirmax, is being modified (adds and removes), and _multiple_
processes are listing it.  In this scenario a lister can still have
unbounded time, but the existing readdir code also has this problem
and fixing that probably requires another patch / approach (such as
reworking the structure of the cache or an approach such as the flag
on per-process dir context).

I think these patches make NFS readdir dramatically better, and they
fix a lot of underlying issues with the existing code, like the cookie
resets when pagecache is dropped, single page rx data problem, etc.
Thank you for doing these patches.

Tested-by: Dave Wysochanski